As is well known, heavy duty road vehicles, such as trailer tractors, employ compressed air brake systems. Such heavy duty vehicles are commonly equipped with a turbocharged or supercharged internal combustion engine and the pneumatic brake system obtains its supply of air from the air intake manifold of the vehicle engine where the air is already at superatmospheric pressure, say 25 psig, as a result of supercharging. Such pneumatic or air braking systems generally involve an air compressor operable in normal or unloading modes, a governor for controlling the operating cycle of the air compressor, an air dryer and a reservoir for holding pressurized air for delivery to the vehicle brakes.
When the pressure in the air reservoir falls below a predetermined minimum pressure, say about 100 psig, the compressor governor then causes the air compressor to operate in a normal or loading mode to compress air. The compressed air from the air compressor during its normal or compressing cycle is passed through an air dryer to remove moisture and then into the air brake reservoir or reservoirs. When the desired pressure, say about 120 psig, is attained in the pressurized air reservoir, the compressor governor functions to cause the air compressor to unload (unloading mode). When the compressor operates in the unloading mode and without the improvement of the present invention, the intake air from the vehicle engine manifold passes through the compressor, but is not compressed therein. It then passes through the air dryer and is then discharged into the atmosphere via an air dryer purge valve. The free or unrestricted flow of intake air from the engine manifold through the compressor and air dryer during the compressor unloading cycle is wasteful and results in a loss of engine power and efficiency. This is particularly significant if the intake air from the vehicle engine manifold is under pressure as is the case with turbocharged or supercharged engines.